The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to an apparatus to acquire high resolution MR images at a site of carotid artery bifurcation. The invention is further directed to an imaging coil, applicable with different patient sizes and contours, capable of acquiring MR data with sufficient penetration to capture contrast of a plaque formed of a thin fibrous membrane covering a large lipid. The present invention may therefore be advantageously implemented in the detection of carotid atherosclerosis—a leading cause of acute stroke.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Noninvasive imaging of carotid artery bifurcation, which is prone to plaque formation due to turbulent blood flow, is of great neurological interest, since carotid atherosclerosis is a leading cause of acute stroke. The vulnerable plaque is typically formed from a thin fibrous membrane of less than 65 μm, covering a large, possibly necrotic, lipid core. A necrotic core of the plaque can be on the order of 1 mm2. A rupture of this thin fibrous cap can expose the thrombogenic lipid core of the atheroma to flowing blood, exposing a patient to a risk of thrombosis.
For accurate determination of vulnerable atheromas, extremely high spatial resolution is required to measure the necrotic core, thrombolytic element, and possibly the thickness of the fibrous membrane. Detecting plaque inflammation due to macrophage infiltration may also provide important information in the diagnosis and treatment of vulnerable plaques. Since plaque components have sub-millimeter dimensions, MR images with high spatial resolution and sufficient signal-to-noise ratio (SNR) at the site of carotid artery bifurcation, which, on average, is 3.5 cm away from the surface of human neck, are required to characterize the vulnerable plaque to determine the probability of rupture. A resolution of about 50-300 μm in-plane is desirable in most cases.
Conventional approaches for local imaging have included using relatively small local coils. Small local coils are known to produce high resolution images with good SNR; however, signal penetration of known small coils is generally not sufficient to provide high resolution images at the site of carotid artery bifurcation.
Furthermore, the site of carotid artery bifurcation varies across patient populations. For example, the location of the carotid artery bifurcation may be closer to the head of one patient and closer to the base of the neck in another patient. Moreover, to increase patient throughput, a large field-of-view (FOV) in the superior—inferior (SI) direction is necessary to ensure capturing the carotid artery bifurcation within the FOV during imaging. Known local imaging coils lack the patient-to-patient variability desired for carotid artery bifurcation imaging. Moreover, these known local imaging coils do not provide the desired balance between SNR and penetration desired for carotid artery bifurcation imaging. Furthermore, in producing maps of the regions, which can be used for sizing and characterization of the local lesions, it is important to produce little fall off in the regions of interest. Smaller coils can produce depth fall off of CNR (contrast to noise ratio) as well as SNR (signal to noise ratio). This can lead to parametric deviations such as seen in T2 and T2* differences in images, which can produce misleading results about the stratification of the plaques. Additionally, a large enough field of view in the SI direction is required due to possible surgical planning which may occur from the images. Surgical planning requires obtaining certain anatomical landmarks in the images relative to the plaques (i.e. location of certain nerves and spine processes before a carotid endarterectomy).
It would therefore be desirable to have an apparatus for acquiring MR data with a high spatial resolution and an SNR at the site of carotid artery bifurcation that is applicable with subjects of varying sizes.